Aqueous fluoropolymer dispersion stabilized with amine oxide surfactant and process for making coagulated fluoropolymer resin

ABSTRACT

An aqueous fluoropolymer dispersion comprising an aqueous medium, fluoropolymer particles, and an amine oxide surfactant. Coagulated fluoropolymer resin is produced from the dispersion by adding an acidic reagent in sufficient amount to cause coagulation and separation of the coagulated fluoropolymer resin from the aqueous medium. Another particulate component such as particular polymer, filler, pigment, solid lubricant, etc., may be added to the dispersion and co-coagulated to form a mixture of coagulated fluoropolymer resin and particulate component.

FIELD OF INVENTION

The present invention relates to aqueous fluoropolymer dispersion andmore particularly relates to aqueous fluoropolymer dispersion comprisingamine oxide surfactant and a process for making coagulated fluoropolymerresin from such dispersion.

BACKGROUND OF THE INVENTION

Fluoropolymer dispersions have a wide variety of end uses. For coatingend uses, coating compositions containing fluoropolymer dispersion areapplied to any of a variety of substrates in order to confer release,chemical and heat resistance, corrosion protection, cleanability, lowflammability, and weatherability. Fluoropolymer dispersions for coatinguse are usually in a concentrated form and typically are stabilized witha significant quantity of a nonionic surfactant such as an alkyl phenolethoxylate or an aliphatic alcohols ethoxylate as taught in U.S. Pat.No. 3,037,953 to Marks et al., U.S. Pat. No. 6,153,688 to Miura et al.,and U.S. 2003/0130393 to Cavanaugh et al.

For some specialized fluoropolymer dispersions, the presence of nonionicsurfactants such as alkyl phenol ethoxylates or aliphatic alcoholsethoxylates is not desirable. One such type of dispersion is generallyreferred to as “raw” dispersion (also referred to as “unstabilized”dispersion) because no surfactant other than the fluorosurfactant usedin polymerization is added. In some processes, raw dispersion iscoagulated to obtain coagulated fluoropolymer resin which, in the caseof polytetrafluoroethylene (PTFE) fluoropolymer, is referred to as finepowder. In other processes, the aqueous fluoropolymer dispersion ismixed with other materials, often in dispersion or slurry form, such asparticulate polymers, fillers, pigments, solid lubricants, etc. and thenthe fluoropolymer is co-coagulated together with the other material.Conventional nonionic surfactants such as alkyl phenol ethoxylates oraliphatic alcohol ethoxylates are not used in coagulation processesbecause they generally confer high stability to the dispersionpreventing or making coagulation difficult. In some processes wherecoagulation does succeed, the result is an undesirable sticky orfibrillated product which is difficult to process into a finishedarticle.

In the manufacture of fluoropolymer dispersions, anionicfluorosurfactant is typically used as a polymerization aid in thedispersion polymerization process, the anionic fluorosurfactantfunctioning as a non-telogenic dispersing agent. For example, an earlydescription of this use of anionic fluorosurfactant in dispersionpolymerization is found in U.S. Pat. No. 2,559,752 to Berry. Because ofenvironmental concerns and because anionic fluorosurfactants areexpensive, processes have been developed for reducing and recoveringanionic fluorosurfactant from aqueous fluoropolymer dispersions.

One common method to reduce anionic fluorosurfactant is by adsorptiononto an ion exchange resin as taught in U.S. Pat. No. 3,536,643(Stryker); U.S. Pat. No. 3,882,153 (Seki et al); U.S. Pat. No. 4,282,162(Kuhls) and U.S. Pat. No. 6,833,403 (Bladel et al.) For fluorosurfactantreduction, fluoropolymer dispersions are generally stabilized with thesame nonionic surfactants that are employed for coating end uses, i.e.,alkyl phenol ethoxylates or aliphatic alcohol ethoxylates. However, ifit is attempted to use an anion exchange process for fluorosurfactantreduction of raw fluoropolymer dispersions, premature coagulation of thedispersion will result. Such premature coagulation of the raw dispersionoccurs because the dispersion is stabilized only by the presence ofanionic fluorosurfactant which is being removed in the anion exchangeprocess.

Improved aqueous fluoropolymer dispersions are desired which areespecially suitable for end use applications which produce coagulatedfluoropolymer resin from the dispersion.

BRIEF SUMMARY OF THE INVENTION

The invention is based on the discovery that amine oxide surfactantsprovide good stabilization for fluoropolymer dispersions and that acidicreagents when added to the dispersions reduce dispersion stability andenable the fluoropolymer dispersions to be coagulated.

In accordance with the invention, an aqueous fluoropolymer dispersion isprovided which comprises an aqueous medium, fluoropolymer particles, andan amine oxide surfactant of the formula:(R¹)(R²)(R³)N→O

wherein R¹ is radical of the formula:R⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)—

wherein R⁴ is a saturated or unsaturated, branched or unbranched, cyclicor acyclic, alkyl, hydroxyalkyl, ether or hydroxyether radical having 1to 20 carbon atoms, X is an O, NH or NR⁵, a and b are 0 or 1 with theproviso that a+b=1, and n is 2-6;

wherein R² and R³ are independently selected from saturated orunsaturated, branched or unbranched, cyclic or acyclic, alkyl,hydroxyalkyl, ether or hydroxyether radical having 1 to 10 carbon atomsoptionally substituted with halogen;

R⁵ is selected from saturated or unsaturated, branched or unbranched,cyclic or acyclic, alkyl, hydroxyalkyl, ether or hydroxyether radicalhaving 1 to 10 carbon atoms optionally substituted with halogen or anN-oxylamino group; and

wherein that R² and R³ may be joined by a chemical bond to form a ring.

The invention also provides a process for producing a coagulatedfluoropolymer resin comprising providing an aqueous fluoropolymerdispersion comprising an aqueous medium, fluoropolymer particles, and anamine oxide surfactant. An acidic reagent is added to the dispersion insufficient amount to coagulate the fluoropolymer particles to producecoagulated fluoropolymer resin. The coagulated fluoropolymer resin isthen separated from the aqueous medium. In a preferred form of theprocess of the invention, process further comprises agitating thedispersion. Preferably, the process further comprises drying saidcoagulated fluoropolymer resin.

In accordance with another preferred form of the invention, the processfurther comprises adding a particulate component such as a particulatepolymer, filler, pigment, solid lubricant, etc., to the aqueousdispersion prior to adding the acidic reagent. In this form of theinvention, the acidic reagent causes co-coagulation of the fluoropolymerparticles and the particulate component. Preferably, the particulatecomponent is added to the dispersion as a dispersion or a slurry in anaqueous medium.

DETAILED DESCRIPTION OF THE INVENTION Fluoropolymers

The aqueous fluoropolymer dispersion in accordance with the presentinvention is made by dispersion polymerization (also known as emulsionpolymerization). Fluoropolymer dispersions are comprised of particles ofpolymers made from monomers wherein at least one of the monomerscontains fluorine, i.e., a fluorinated monomer, preferably an olefinicmonomer with at least one fluorine or a perfluoroalkyl group attached toa doubly-bonded carbon. The fluorinated monomer used in the process ofthis invention is preferably selected from the group consisting oftetrafluoroethylene (TFE), hexafluoropropylene (HFP),chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluoroalkyl ethylene, fluorovinyl ethers,vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD) andperfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD). A preferredperfluoroalkyl ethylene monomer is perfluorobutyl ethylene (PFBE).Preferred fluorovinyl ethers include perfluoro(alkyl vinyl ether)monomers (PAVE) such as perfluoro(propyl vinyl ether) (PPVE),perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether)(PMVE). Non-fluorinated olefinic comonomers such as ethylene andpropylene can be copolymerized with fluorinated monomers. A preferredclass of fluoropolymers are homopolymers and copolymers oftetrafluoroethylene (TFE).

Preferred fluoropolymer particles in the dispersion employed in thisinvention are non-melt-processible particles of polytetrafluoroethylene(PTFE) including modified PTFE which is not melt-processible.Polytetrafluoroethylene (PTFE) refers to the polymerizedtetrafluoroethylene by itself without any significant comonomer present.Modified PTFE refers to copolymers of TFE with such small concentrationsof comonomer that the melting point of the resultant polymer is notsubstantially reduced below that of PTFE. The concentration of suchcomonomer is preferably less than about 1 wt %, more preferably lessthan about 0.5 wt %. A minimum amount of at least about 0.05 wt % ispreferably used to have significant effect. The modified PTFE preferablycontains a comonomer modifier which improves film forming capabilityduring baking (fusing), such as perfluoroolefin, notablyhexafluoropropylene (HFP) or perfluoro(alkyl vinyl)ether (PAVE), wherethe alkyl group contains 1 to 5 carbon atoms, with perfluoro(ethylvinyl)ether (PEVE) and perfluoro(propyl vinyl)ether (PPVE) beingpreferred. Chlorotrifluoroethylene (CTFE), perfluorobutyl ethylene(PFBE), or other monomer that introduces bulky side groups into themolecule are also included. In this preferred form of the invention, thePTFE or modified PTFE typically has a melt creep viscosity of at least1×10⁸ Pa·s. The resins in the dispersion used in this form of theinvention when coagulated and dried are thus non-melt-processible.

By non-melt-processible, it is meant that no melt flow is detected whentested by the standard melt viscosity determining procedure formelt-processible polymers. This test is according to ASTM D-1238-00modified as follows: The cylinder, orifice and piston tip are made ofcorrosion resistant alloy, Haynes Stellite 19, made by Haynes StelliteCo. The 5.0 g sample is charged to the 9.53 mm (0.375 inch) insidediameter cylinder which is maintained at 372° C. Five minutes after thesample is charged to the cylinder, it is extruded through a 2.10 mm(0.0825 inch diameter), 8.00 mm (0.315 inch) long square-edge orificeunder a load (piston plus weight) of 5000 grams. This corresponds to ashear stress of 44.8 KPa (6.5 pounds per square inch). No melt extrudateis observed.

In one preferred embodiment of the invention, the fluoropolymerparticles are of high molecular weight polytetrafluoroethylene (PTFE) ormodified polytetrafluoroethylene which, in fine powder form, are usefulfor the manufacture of paste extruded shapes that can be stretchedrapidly in the unsintered state to form a high tensile strength PTFEfiber or expanded PTFE sheets or membranes. Processes for makingdispersions containing PTFE or modified PTFE of this type are disclosedin Malhotra, U.S. Pat. No. 4,576,869, and Jones, U.S. Pat. No. 6,177,533B1.

The preferred non-melt-processible PTFE or modified PTFE have a standardspecific gravity (SSG) of about 2.13 to about 2.50. Preferably, the SSGis less than about 2.40, more preferably less than about 2.30, and mostpreferably less than about 2.25. The SSG is generally inverselyproportional to the molecular weight of PTFE or modified PTFE.

The fluoropolymer particles in the dispersion used in this inventionhave a number average particle size of about 10 nm to about 400 nm,preferably, about 100 nm to about 350 nm.

The invention is also useful for dispersions of melt-processiblefluoropolymers. By melt-processible, it is meant that the polymer can beprocessed in the molten state (i.e., fabricated from the melt intoshaped articles such as films, fibers, and tubes etc. that exhibitsufficient strength and toughness to be useful for their intendedpurpose). Examples of such melt-processible fluoropolymers includehomopolymers such as polychlorotrifluoroethylene or copolymers oftetrafluoroethylene (TFE) and at least one fluorinated copolymerizablemonomer (comonomer) present in the polymer usually in sufficient amountto reduce the melting point of the copolymer substantially below that ofTFE homopolymer, polytetrafluoroethylene, e.g., to a melting temperatureno greater than 315° C.

A melt-processible TFE copolymer typically incorporates an amount ofcomonomer into the copolymer in order to provide a copolymer which has amelt flow rate (MFR) of about 1-100 g/10 min as measured according toASTM D-1238 at the temperature which is standard for the specificcopolymer. Preferably, the melt viscosity is at least about 10² Pa·s,more preferably, will range from about 10² Pa·s to about 10⁶ Pa·s, mostpreferably about 10³ to about 10⁵ Pa·s measured at 372° C. by the methodof ASTM D-1238 modified as described in U.S. Pat. No. 4,380,618.Additional melt-processible fluoropolymers are the copolymers ofethylene or propylene with TFE or CTFE, notably ETFE, ECTFE and PCTFE.

A preferred melt-processible copolymer for use in the practice of thepresent invention comprises at least about 40-98 mol %tetrafluoroethylene units and about 2-60 mol % of at least one othermonomer. Preferably the other monomer is a perfluorinated monomer.Preferred comonomers with TFE are perfluoroolefin having 3 to 8 carbonatoms, such as hexafluoropropylene (HFP), and/or perfluoro(alkyl vinylether) (PAVE) in which the linear or branched alkyl group contains 1 to5 carbon atoms. Preferred PAVE monomers are those in which the alkylgroup contains 1, 2, 3 or 4 carbon atoms, and the copolymer can be madeusing several PAVE monomers. Preferred TFE copolymers include FEP(TFE/HFP copolymer), PFA (TFE/PAVE copolymer), TFE/HFP/PAVE wherein PAVEis PEVE and/or PPVE, MFA (TFE/PMVE/PAVE wherein the alkyl group of PAVEhas at least two carbon atoms) and THV (TFE/HFP/VF2).

A typical process for the aqueous dispersion polymerization of preferredPTFE polymer is a process wherein TFE vapor is fed to a heated reactorcontaining fluorosurfactants, paraffin wax and deionized water. A chaintransfer agent may also be added if it is desired to reduce themolecular weight of the PTFE. A free-radical initiator solution is addedand, as the polymerization proceeds, additional TFE is added to maintainthe pressure. The exothermic heat of reaction is removed by circulatingcooling water through the reactor jacket. After several hours, the feedsare stopped, the reactor is vented and purged with nitrogen, and the rawdispersion in the vessel is transferred to a cooling vessel. Paraffinwax is removed and the dispersion is isolated and stabilized withdispersing agent.

The fluorosurfactant used in the manufacture of the dispersion is anon-telogenic, anionic dispersing agent, soluble in water and comprisingan anionic hydrophilic group and a hydrophobic portion. Preferably, thehydrophobic portion is an aliphatic fluoroalkyl group containing atleast four carbon atoms and bearing fluorine atoms and having no morethan two carbon atoms not bearing fluorine atoms adjacent to thehydrophilic group. These fluorosurfactants are used as a polymerizationaid for dispersing and, because they do not chain transfer, they do notcause formation of polymer with undesirable short chain length. Anextensive list of suitable fluorosurfactants is disclosed in U.S. Pat.No. 2,559,752 to Berry. Preferably, the fluorosurfactant is aperfluorinated carboxylic or sulfonic acid having 6-10 carbon atoms andis typically used in salt form. Suitable fluorosurfactants are ammoniumperfluorocarboxylates, e.g., ammonium perfluorocaprylate or ammoniumperfluorooctanoate. The fluorosurfactants are usually present in theamount of 0.02 to 1 wt % with respect to the amount of polymer formed.The fluorinated surfactant is used to aid the polymerization process butthe amount remaining in the dispersion is significantly reduced as willbe explained below.

The initiators preferably used to make dispersion of this invention arefree radical initiators. They may be those having a relatively longhalf-life, preferably persulfates, e.g., ammonium persulfate orpotassium persulfate. To shorten the half-life of persulfate initiators,reducing agents such as ammonium bisulfite or sodium metabisulfite, withor without metal catalysis salts such as Fe (III), can be used.Alternatively, short half-life initiators such as potassiumpermanganate/oxalic acid can be used.

In addition to the long half-life persulfate initiators, small amountsof short chain dicarboxylic acids such as succinic acid or initiatorsthat produce succinic acid such as disuccinic acid peroxide (DSP) may bealso be added in order to reduce coagulum

The dispersion polymerization of melt-processible copolymers is similarexcept that comonomer in significant quantity is added to the batchinitially and/or introduced during polymerization. Chain transfer agentsare typically used in significant amounts to decrease molecular weightto increase melt flow rate.

Amine Oxide Surfactants

Amine oxide surfactants are employed in accordance with the invention toprovide stabilization for the aqueous fluoropolymer dispersions and tobe selectively destabilized by the addition of an acidic reagent. Inaqueous fluoropolymer dispersion compositions in accordance with theinvention and in accordance with preferred amine oxide surfactants foruse in the process of the invention, amine oxide surfactants of theformula are employed:(R¹)(R²)(R³)N→O

wherein R¹ is radical of the formula:R⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)

wherein R⁴ is a saturated or unsaturated, branched or unbranched, cyclicor acyclic, alkyl, hydroxyalkyl, ether or hydroxyether radical having 1to 20 carbon atoms, X is an O, NH or NR⁵, a and b are 0 or 1 with theproviso that a+b=1, and n is 2-6;

wherein R² and R³ are independently selected from saturated orunsaturated, branched or unbranched, cyclic or acyclic, alkyl,hydroxyalkyl, ether or hydroxyether radical having 1 to 10 carbon atomsoptionally substituted with halogen;

R⁵ is selected from saturated or unsaturated, branched or unbranched,cyclic or acyclic, alkyl, hydroxyalkyl, ether or hydroxyether radicalhaving 1 to 10 carbon atoms optionally substituted with halogen or anN-oxylamino group; and

wherein that R² and R³ may be joined by a chemical bond to form a ring.

If R², R³, R⁴ and R⁵ have halogen substitutions, preferably halogensubstitutions are limited such that no more than about 70% of the atomsattached to carbon atoms of the radical are halogen atoms, morepreferably no more than about 50% are halogen atoms. Most preferably,R², R³, R⁴ and R⁵ are not halogen substituted.

If R⁵ is substituted with N-oxylamino, groups bonded to the nitrogenatom preferably have 1 to 10 carbon atoms.

In preferred surfactants, R1 is a radical of the formulaR⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)

wherein R⁴ comprises alkyl having 1-20 carbon atoms, X is NH, a and bare 0 or 1 with the proviso that a+b=1, and n is 2-4;

In more preferred surfactants, R1 is a radical of the formula:R⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)

wherein R⁴ comprises alkyl having 5-20 carbon atoms, X is NH, a and bare 0 or 1 with the proviso that a+b=1, and n is 3.

In preferred surfactants, R² and R³ in the formula above areindependently selected from saturated or unsaturated, branched orunbranched, cyclic or acyclic, alkyl or hydroxyalkyl radical having 1 to4 carbon atoms.

In more preferred surfactants, R² and R³ in the formula above are eachindependently selected from alkyl or hydroxyalkyl radicals having 1 to 2carbon atoms.

Preferred surfactants useful for the practice of the present inventionare cocoamidopropyl dimethyl amine oxide, 2-ethylhexylamidopropyldimethyl amine oxide, and octylamidopropyl dimethyl amine oxide.

Dispersions

To produce dispersion in accordance with the invention, amine oxidesurfactant as described above is added in sufficient quantity to thedispersion after polymerization to stabilize the dispersion for theintended processing. Preferably, the stability provided enables thedispersion to be treated to reduce fluorosurfactant content such as byanion exchange as discussed hereinafter. In addition, for use inaccordance with the process of the invention, the amount and type ofamine oxide surfactant is selected to enable the dispersion to becoagulated by addition of an acidic reagent.

The amine oxide surfactant can be added to the dispersion anytime afterpolymerization but prior to fluorosurfactant reduction as discussed inmore detail hereinafter. Usually, the amine oxide surfactant is addedprior to concentration if concentration is to be performed.Concentration is discussed in more below.

The aqueous dispersions in accordance with the invention preferablyrange in fluoropolymer solids content from about 10 to about 70 wt %,more preferably 25 to about 70 wt %. The amount of amine oxidesurfactant is preferably selected based on the amount of fluoropolymersolids. The aqueous fluoropolymer dispersions in accordance with theinvention preferably contain amine oxide surfactant in an amount ofabout 0.05 to about 15 wt % based on the weight of fluoropolymer solids.More preferably, the aqueous fluoropolymer dispersion is present in anamount of about 0.1 to about 10 wt %, most preferably 0.5 to about 5 wt%, based on the weight of fluoropolymer solids. For coagulation enduses, the exact amount of amine oxide surfactant in a particulardispersion should be selected to provide stability during processing butpermit coagulation when desired. As discussed in more detailhereinafter, the amount of amine oxide surfactant may need to beadjusted for end uses in which particulate component is to beco-coagulated together with the fluoropolymer.

The size of the fluoropolymer particles in the aqueous fluoropolymerdispersion is determined by the polymerization procedure used to makethe dispersion. Preferred dispersions have a number average particlesize of about 10 to about 400 nm.

The dispersions in accordance with the invention preferably have reducedanionic fluorosurfactant content, preferably at a level no greater thanabout 300 ppm, more preferably no greater than about 100 ppm, mostpreferably no greater than about 50 ppm.

Dispersion Shear Stability—Gel Time

The dispersion preferably has a Gel Time of at least 100 seconds asdetermined by the Gel Time test described in the Test Methods of thisapplication. Gel Time is a measurement of resistance of the dispersionto coagulation under high shear conditions and thus is an indicator ofthe stability of the dispersion during processing which subjects thedispersion to shear. Although affected by a variety of factors includingsolids content, pH, molecular weight of the polymer, polymer particlemorphology, other materials in the dispersion, etc., a Gel Time of atleast 100 indicates that the amine oxide surfactant is functioning tostabilize the polymer sufficiently for normal handling and processing,e.g., is sufficiently stabilized for fluorosurfactant removal in ananion exchange column. More preferably, the Gel Time is at least about300 seconds, even more preferably at least about 500 seconds, even morepreferably at least about 1000 seconds, and most preferably at leastabout 1500 seconds. A preferred range of Gel Time provided by thepresent invention is about 100 seconds to about 2000 seconds. Inaccordance with a preferred form of the invention, the dispersioncontains less that about 300 ppm fluorosurfactant based on the weight ofthe dispersion and has the Gel Times as indicated above. Preferably, theGel Times described above are observed when the fluorosurfactant contentis less that about 100 ppm, most preferably less that about 50 ppm.

The preferred dispersions in accordance with the invention also havelong storage stability and can be stored at least about 2 weeks withoutany significant coagulation or degradation. More preferably, thedispersions are stable for storage at least about 2 months.

Fluorosurfactant Reduction

In the practice of the present invention, it is preferable for theanionic fluorosurfactant content of the aqueous fluoropolymer dispersionto be reduced to a predetermined level, preferably a level no greaterthan about 300 ppm, more preferably no greater than about 100 ppm, mostpreferably no greater than about 50 ppm.

The fluorosurfactant content can be reduced by any of a variety ofprocedures as known in the art. With stabilization being provided byamine oxide surfactant, the fluorosurfactant can be advantageouslyremoved by adsorption onto an anion exchange resin without coagulationof the dispersion occurring. Any of a variety of techniques which bringthe dispersion in contact with the anion exchange resin can be used tocarry out the ion exchange of the process. For example, the process canbe carried out by addition of ion exchange resin bead to the dispersionin a stirred tank, in which a slurry of the dispersion and resin isformed, followed by separation of dispersion from the anion exchangeresin beads by filtration. Another suitable method is to pass thedispersion through a fixed bed of anion exchange resin instead of usinga stirred tank. Flow can be upward or downward through the bed and noseparate separation step is needed since the resin remains in the fixedbed.

The contacting of the dispersion is performed at a temperature which issufficiently high to facilitate the rate of ion exchange and to reducethe viscosity of the dispersion but being below a temperature at whichthe resin degrades at a detrimentally high rate or a viscosity increasein observed. Upper treatment temperature will vary with the type ofpolymer and amine oxide surfactant employed. Typically, temperatureswill be between 20° C. and 80° C.

The fluorosurfactant can be recovered from the anion exchange resin ifdesired or the resin with the fluorosurfactant can be disposed of in anenvironmentally acceptable method, e.g., by incineration. If it isdesired to recover the fluorosurfactant, the fluorosurfactant may beremoved from resin by elution. Elution of fluorosurfactant adsorbed onthe anion exchange resin is readily achieved by use of ammonia solutionas demonstrated by Seki in U.S. Pat. No. 3,882,153, by a mixture ofdilute mineral acid with organic solvent (e.g., HCl/ethanol) asdemonstrated by Kuhls in U.S. Pat. No. 4,282,162, or by strong mineralacids such as sulfuric acid and nitric, transferring the adsorbedfluorinated carboxylic acid to the eluent. The fluorosurfactant in theeluent in high concentration can easily be recovered in the form of apure acid or in the form of salts by common methods such asacid-deposition, salting out, and other methods of concentration, etc.

Ion Exchange Resins

The ion exchange resins for use in accordance with reducing thefluorosurfactant content of the aqueous dispersion used in the presentinvention include anionic resins but can also include other resin typessuch as cationic resins, e.g., in a mixed bed. The anionic resinsemployed can be either strongly basic or weakly basic. Suitable weaklybasic anion exchange resins contain primary, secondary amine, ortertiary amine groups. Suitable strongly basic anion exchange resincontain quaternary ammonium groups. Although weakly basic resins areuseful because they can be regenerated more easily, strongly basisresins are preferred when it is desired to reduce fluorosurfactant tovery low levels and for high utilization of the resin. Strongly basicion exchange resins also have the advantage of less sensitivity to thepH of the media. Strong base anion exchange resins have an associatedcounter ion and are typically available in chloride or hydroxide formbut are readily converted to other forms if desired. Anion exchangeresins with hydroxide, chloride, sulfate, and nitrate can be used forthe removal of the fluorosurfactant but anion exchange resins in theform of hydroxide are preferred to prevent the introduction ofadditional anions and to increase pH during anion exchange because ahigh pH, i.e., greater than 9, is desirable in the product prior toshipping to inhibit bacterial growth. Examples of suitablecommercially-available strong base anion exchange resins with quaternaryammonium groups with a trimethylamine moiety include DOWEX® 550A, USFilter A464-OH, SYBRON M-500-OH, SYBRON ASB1-OH, PUROLITE A-500-OH,Itochu TSA 1200, AMBERLITE® IR 402. Examples of suitablecommercially-available strong base anion exchange resins with quaternaryammonium groups with a dimethyl ethanol amine moiety include US FilterA244-OH, AMBERLITE® 410, DOWEX® MARATHON A2, and DOWEX® UPCORE Mono A2.

Ion exchange resin used to reduce fluorosurfactant for use in theprocess of the present invention is preferably monodisperse. Preferably,the ion exchange resin beads have a number average size distribution inwhich 95% of the beads have a diameter within plus or minus 100 μm ofthe number average bead diameter.

Concentration

If concentration of the dispersions of this invention is desired, any ofa variety of known methods can be used. If an amine oxide surfactant isemployed which has a cloud point in a practical temperature range forconcentration, i.e., about 30° C. and about 90° C., concentration can beperformed as taught in Marks et al., U.S. Pat. No. 3,037,953. Otherknown methods can be practiced if desired and can be used when the amineoxide surfactant does not have a suitable cloud point. Ultrafiltrationas taught in Kuhls, U.S. Pat. No. 4,369,266, can be used. Anothersuitable method is concentration using acrylic polymers of high acidcontent as described in U.S. Pat. No. 5,272,186 to Jones.

Process to Produce Coagulated Fluoropolymer Resin

The process of the invention produces coagulated fluoropolymer resinfrom aqueous fluoropolymer dispersions containing amine oxidesurfactant. An acidic reagent, preferably a mineral acid or strongorganic acid, is added to the dispersion in sufficient amount tocoagulate the fluoropolymer particles to produce coagulatedfluoropolymer resin. The coagulated fluoropolymer resin is thenseparated from the aqueous medium. In a preferred form of the process ofthe invention, process further comprises agitating the dispersion.Preferably, the process further comprises drying said coagulatedfluoropolymer resin.

Coagulation of fluoropolymer dispersion in accordance with the inventionpreferably is accomplished by introducing the dispersion into a vesselequipped with an agitator suitable for providing suitable shear to thedispersion for mixing and assisting with coagulation. Turbine agitatorsare suitable for this purpose and may have pitched blades with thevertical displacement given to the dispersion being either up or down.If air entrainment is be minimized, vertical displacement upwards isgenerally desirable. With amine oxide surfactants, and particularly withthe preferred amine oxide surfactants, the stability of the dispersiondecreases with decreasing pH. It is preferably for the pH to bedecreased together with suitable agitation for mixing. In addition, itis preferable for the acid to be added over time to allow good dispersalof the acid and it is usually desirable for the pH of the dispersion isgradually dropped. Preferably, the pH is adjusted to be acidic, i.e., apH in the range of about 7 to about 0. Preferably, the pH is adjust toless than about 7, more preferably less than about 5, most preferablyless than about 3. When the desired pH has been achieved, the agitationrate can be increased to effect the coagulation of the dispersion,optionally with the addition of more acid either continuously or byaliquots. Usually upon coagulation, the dispersion will separate into afloating polymer layer and a relatively clear water layer. The aqueousmedium can be drained off, siphoned off, or otherwise separated from thepolymer. Additional acidified water or fluoropolymer surface wettingsolvents such as the lower alcohols can be contacted with the polymerone or more times to remove more of the adsorbed amine oxide surfactantif desired. Once a suitable amount of the amine oxide surfactant isremoved, the polymer is separated from the bulk of water and or solvent,and the polymer is dried and can be used as desired.

In accordance with another preferred form of the invention, the processfurther comprises adding a particulate component such as a particulatepolymer, filler, pigment, solid lubricant, etc., to the aqueousdispersion prior to adding the acidic reagent. In this form of theinvention, the acidic reagent causes co-coagulation of the fluoropolymerparticles and the particulate component. This form of the invention isadvantageously used when it is desired to have an intimate mixture ofthe fluoropolymer and the particulate component. In such intimatemixtures, the particulate component is preferably uniformly distributedin the coagulated fluoropolymer resin such that the mixture ishomogeneous. Preferred co-coagulated materials made in this process canbe processed in, for example, paste extrusion processes similar to finepowder resin without additives.

For co-coagulation of the dispersion with another particulate componentsuch as a particulate polymer, filler, pigment, solid lubricant, etc.,it is usually desirable for the selected particulate component to beprovided as a dispersion or as a slurry with water, preferably watercontaining surfactant. Solvents may also be present also is desired.Particulate components often have large and adsorptive surface areasthat can absorb a surfactant and destabilize the dispersion causingpremature co-coagulation, i.e., before intimate mixing is achieved.Accordingly, it may be desirable to treat the particulate component toprevent premature co-coagulation such as by contacting the particulatecomponent with the same or different surfactant as is contained in thefluoropolymer dispersion, i.e., amine oxide. One way to achieve this isto provide an particulate component slurry or dispersion having thenearly the same or lower surface tension than that of the fluoropolymerdispersion. As discussed above for coagulation of the fluoropolymeralone, co-coagulation together the particulate component with can beachieved, in accordance with the invention, by addition of an acidicreagent. Preferably, the manner of addition of the acidic reagent andthe pH range employed is the same as discussed above for the coagulationof the fluoropolymer resin alone. Preferably, agitation is also employedas discussed above. Ultimately, after washing if desired, theco-coagulated mixture is dried or otherwise used as desired.

Test Methods

Solids content of raw (as polymerized) fluoropolymer dispersion aredetermined gravimetrically by evaporating a weighed aliquot ofdispersion to dryness, and weighing the dried solids. Solids content isstated in weight % based on combined weights of PTFE and water.Alternately solids content can be determined by using a hydrometer todetermine the specific gravity of the dispersion and then by referenceto a table relating specific gravity to solids content. (The table isconstructed from an algebraic expression derived from the density ofwater and density of as polymerized PTFE.)

Number average dispersion particle size on raw dispersion is measured byphoton correlation spectroscopy.

Standard specific gravity (SSG) of PTFE resin is measured by the methodof ASTM D-4895. If a surfactant is present, it can be removed by theextraction procedure in ASTM-D-4441 prior to determining SSG by ASTMD-4895.

Surfactant Content is calculated based on the amount of amine oxidesurfactant added to the dispersion and is reported as wt % based onfluoropolymer solids.

Fluorosurfactant Content is measured by a GC technique in which thefluorosurfactant is esterified with acidic methanol. Perfluoroheptanoicacid is used as an internal standard. Upon addition of electrolyte andhexane the ester is extracted into the upper hexane layer. The hexanelayer is analyzed by injection onto a glass GC column of 20 ft.×2 mmI.D. packed with 10% OV-210 on 70/80 mesh Chromosorb W.AW.DMCS. held at120 C. The detector is ECD and the carrier gas of 95% argon/5% methanehas a flow rate of 20 to 30 ml/min. Fluorosurfactant content is reportedas wt % based on dispersion weight.

Gel time is measured as the time it takes a dispersion to completely gelin a blender. 200 ml of dispersion is placed in a Waring commercialexplosion resistant blender (Model 707SB, one quart size, run at highspeed, air requirements—10 scfm @ 10 psi, available from Waring of NewHartford, Conn.). This blender has a capacity of 1 liter and has an airpurge for the motor. The dispersion is stirred at the highest speeduntil the dispersion gels. The Gel Time is recorded is seconds. If thedispersion does not gel in ½ hour (1800 seconds), the test is terminatedto avoid damage to the blender. The blender is then completelydisassembled and cleaned after each determination.

EXAMPLES Example 1

This example illustrates a dispersion stabilized with cocoamidopropyldimethyl amine oxide and reduction of fluorosurfactant from thedispersion using an anion exchange procedure.

To a 2 liter glass resin kettle with cover having an internal diameterof 5 inches (12.5 cm) is added 540 ml of a PTFE dispersion of a highmolecular weight resin of 40 wt % fluoropolymer solids having afluorosurfactant content of 1328 ppm and a particle size of 270 nm. Alsoadded to the kettle are 1420 ml of water, 20 ml of concentrated ammoniumhydroxide (30 wt % as NH₃) and 6 g of cocoamidopropyl dimethyl amineoxide supplied by Jeen International as Jeechem 1770 having about 30 wt% active ingredient. The content of amine oxide surfactant is 0.83 wt %based on fluoropolymer solids. Two small ion nylon mesh bags eachcontaining 15 g of A-244 OH Ion Exchange resin supplied by U.S. FilterCorporation attached to a 4 bladed agitator are suspended below theliquid surface. By means of a water bath the mixture was held at 50° C.and the mixture stirred at 20 rpm. After 2 days the dispersion mixturewas removed from the kettle and found to have a fluorosurfactant contentof 49.5 ppm and essentially no coagulum.

Example 2

This example illustrates a dispersion stabilized with2-ethylhexylamidopropyl dimethyl amine oxide and reduction offluorosurfactant from the dispersion using an anion exchange procedure.

The same conditions as example 1 are employed except that 40 ml ofconcentrated ammonium hydroxide (30 wt % as NH₃) is added and 15 g of2-ethylhexylamidopropyl dimethyl amine oxide supplied by IsotetChemical, LLC is used as the surfactant having 42.6 wt % activeingredient. The content of amine oxide surfactant is 2.95 wt % based onfluoropolymer solids. After 31 hours the dispersion is found to contain40.5 ppm fluorosurfactant content and essentially no coagulum.

Example 3

This example illustrates a dispersion stabilized with octylamidopropyldimethyl amine oxide and reduction of fluorosurfactant from thedispersion using an anion exchange procedure.

The same conditions as example 2 are employed except 15 g ofoctylamidopropyl dimethyl amine oxide supplied by Isotet Chemical, LLChaving 44.3 wt % active ingredient is used as the surfactant (3.1 wt %based on fluoropolymer solids) and the temperature is held at 60 degreesC. After 2 days the dispersion is found to contain 18.2 ppmfluorosurfactant and essentially no coagulum.

Example 4

This example illustrates a dispersion stabilized with2-ethylhexylamidopropyl dimethyl amine oxide and coagulation of PTFEresin from the dispersion with sulfuric acid.

800 ml of dispersion prepared as in Example 2 (after fluorosurfactantreduction) is placed in a 2 liter glass resin kettle equipped withstainless steel baffles. The apparatus consists of a 2 liter glass resinkettle of internal diameter of 13 cm, equipped with lid, baffles, a4-bladed turbine agitator attached to a shaft, and a motor that rotatesthe shaft that passes through the lid. The baffles are a unit having anouter diameter of 12.7 cm, which slips into the kettle. The unit iscomprised of 4 baffle blades of height 12.7 cm and 1.25 cm in widtharranged equidistantly around on a wire ring. The agitator has adiameter of 7.5 cm and the blades are 1.4 cm wide. The blades have apitch of 45 degrees and the shaft is rotated such that the agitatorpumps upward. The agitator speed is set to 250 rpm and 3 ml ofconcentrated sulfuric acid is added to reduce the pH to less than about3 and the dispersion begins to coagulate. The agitator speed is raisedto 750 rpm to finish the coagulation. The liquid is decanted and thecoagulated polymer is transferred to a large buchner funnel with fastfiltering paper and the polymer is rinsed with water acidified withsulfuric acid. The polymer is removed and triturated in 2-propanol, themixture again transferred to the buchner funnel to remove the solvent.This is repeated. The polymer is air dried and then dried at atemperature of 150° C. SSG testing yields a dark colored chip andmeasured SSG of 2.1605.

Example 5

This example illustrates a dispersion stabilized with2-ethylhexylamidopropyl dimethyl amine oxide and coagulation of PTFEresin from the dispersion with nitric and phosphoric acid.

The dispersion prepared as in Example 2 (after fluorosurfactantreduction) is coagulated by the same technique as Example 4 except that7 ml of nitric acid is used instead of sulfuric acid. The addition ofthe nitric acid yields a pH of 1-2 measured using pH indicating strips.The SSG chip still has some color but less than Example 4 and SSGmeasured 2.1612.

Using the dispersion of Example 2 (after fluorosurfactant reduction) butacidifying to a pH of 1-2 by use of 5 ml of phosphoric acid yieldspolymer with an SSG of 2.1581.

Example 6

This example illustrates on a larger scale a dispersion stabilized withcocoamidopropyl dimethyl amine oxide and reduction of fluorosurfactantfrom the dispersion using an anion exchange procedure.

To a 50 gallon stainless steel tank with steam heated jacket with lid isadded 212.7 kg of a 23.5 wt % dispersion of high molecular weight resin,1 L of concentrated ammonium hydroxide (30 wt % as NH₃), 1.5 kg ofJeechem 1770 (cocoamidopropyl dimethyl amine oxide, 30 wt %) and 2 kg ofwater. The content of amine oxide surfactant is 0.9 wt % based onfluoropolymer solids. Affixed to the agitator is 28 nylon mesh bagscontaining a total of 4400 g of A-244 ion exchange resin. The agitatorwas turned at 10 rpm and the tank was held at 50 C for 2 days. Thedispersion is discharged from the tank by siphoning. The resultingdispersion has a fluorosurfactant content of 50 ppm.

Example 7

This example illustrates a dispersion stabilized with cocoamidopropyldimethyl amine oxide and coagulation of PTFE resin from the dispersionwith nitric acid at various amine oxide surfactant levels.

The ability to coagulate to a fine powder from a dispersion of about11.5 wt % PTFE solids with various loadings of cocoamidopropyl dimethylamine oxide (AO) is tested using the dispersion prepared as in Example 6using the same apparatus for coagulating fine powder dispersion as usedin Example 4. Nitric acid addition reduces pH to less than 3.

Table 1 below shows the preparation of the dispersion and coagulationconditions.

TABLE 1 Added AO cont Nitric Max Dispersion Water AO (Wt % PTFE AcidAgitation Time (g) (g) (g) Basis) (ml) (RPM) (min) 250 270 0 0.9 9 800 8250 270 0.5 1.1 15 1000 12.5 250 270 1.2 1.5 15 1000 —* *did notcoagulate

1. An aqueous fluoropolymer dispersion comprising an aqueous medium,fluoropolymer particles, and an amine oxide surfactant of the formula:(R¹)(R²)(R³)N→O wherein R¹ is radical of the formula:R⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)— wherein R⁴ is a saturated orunsaturated, branched or unbranched, cyclic or acyclic, alkyl,hydroxyalkyl, ether or hydroxyether radical having 1 to 20 carbon atoms,X is an O, NH or NR⁵, a and b are 0 or 1 with the proviso that a+b=1,and n is 2-6; wherein R² and R³ are independently selected fromsaturated or unsaturated, branched or unbranched, cyclic or acyclic,alkyl, hydroxyalkyl, ether or hydroxyether radical having 1 to 10 carbonatoms optionally substituted with halogen; R⁵ is selected from saturatedor unsaturated, branched or unbranched, cyclic or acyclic, alkyl,hydroxyalkyl, ether or hydroxyether radical having 1 to 10 carbon atomsoptionally substituted with halogen or an N-oxylamino group; whereinthat R² and R³ may be joined by a chemical bond to form a ring; andwherein the aqueous fluoropolymer dispersion has a pH of greater thanabout
 7. 2. The aqueous fluoropolymer dispersion of claim 1 wherein R1is a radical of the formulaR⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)— wherein R⁴ comprises alkyl having1-20 carbon atoms, X is NH, a and b are 0 or 1 with the proviso thata+b=1, and n is 2-4.
 3. The aqueous fluoropolymer dispersion of claim 1wherein R1 is a radical of the formula:R⁴—(C═O)_(a)—X—(C═O)_(b)—(CH₂)_(n)— wherein R⁴ comprises alkyl having5-20 carbon atoms, X is NH, a and b are 0 or 1 with the proviso thata+b=1, and n is
 3. 4. The aqueous fluoropolymer dispersion of claim 1wherein R² and R³ are independently selected from saturated orunsaturated, branched or unbranched, cyclic or acyclic, alkyl orhydroxyalkyl radical having 1 to 4 carbon atoms.
 5. The aqueousfluoropolymer dispersion of claim 1 wherein R² and R³ are eachindependently selected from alkyl or hydroxyalkyl radicals having 1 to 2carbon atoms.
 6. The aqueous fluoropolymer dispersion of claim 1 whereinthe fluoropolymer solids content is about is about 10 to about 70 wt %.7. The aqueous fluoropolymer dispersion of claim 1 wherein said amineoxide surfactant is present in an of amount about 0.05 to about 15 wt %based on the weight of fluoropolymer solids.
 8. The aqueousfluoropolymer dispersion of claim 1 wherein said amine oxide surfactantis present in an amount of about 0.1 to about 10 wt % based on theweight of fluoropolymer solids.
 9. The aqueous fluoropolymer dispersionof claim 1 wherein said amine oxide surfactant is present in an amountof about 0.5 to about 5 wt % based on the weight of fluoropolymersolids.
 10. The aqueous fluoropolymer dispersion of claim 1 wherein saidfluoropolymer comprises a polymer selected from homopolymers andcopolymers of tetrafluoroethylene.
 11. The aqueous fluoropolymerdispersion of claim 10 wherein said fluoropolymer is selected frompolytetrafluoroethylene and modified polytetrafluoroethylene.
 12. Theaqueous fluoropolymer dispersion of claim 10 wherein said fluoropolymeris a melt-processible copolymer comprising at least about 40-98 mol %tetrafluoroethylene units and about 2-60 mol % of at least one othermonomer.
 13. The aqueous fluoropolymer dispersion of claim 12 whereinsaid at least one other monomer comprises at least one perfluorinatedmonomer.
 14. The aqueous fluoropolymer dispersion of claim 1 whereinsaid fluoropolymer particles have a number average particle size ofabout 10 to about 400 nm.